The Future of Fracture Acidizing: A Low pH, Robust, and Residue-Free Crosslinked Fracturing Fluid

摘要

Fracture acidizing continues to be an effective process to enhance production of carbonate formations. To help achieve a successful fracture acidizing treatment, three fundamental issues should be addressed: fluid- loss control, reactivity control, and conductivity generation. Crosslinked fracturing fluids are pumped at the forefront of an acidizing design to initiate the fracture profile and to reduce increased fluid loss caused by acid interactions within the formation. The current industry standardized guar/borate crosslinked fracturing fluids leave insoluble residues, which significantly reduce production. This paper introduces a low pH, robust, and residue-free fracturing fluid for fracture acidizing. This new fluid is a polysaccharide-based crosslinked fracturing fluid system that leaves little-to-no residue upon breaking (<1%). This unique property facilitates improvements in well cleanup and increased hydrocarbon production by eliminating much of the insoluble residue found in traditional fracturing fluids. Typically, a higher pH fracturing fluid is used with acidizing systems because of its temperature tolerance at bottomhole conditions. However, these high-pH fracturing fluids can prematurely lose their crosslink when coming into contact with acid. Therefore, typical designs are based on the assumption of limited intermingling between the acid systems and crosslinked fluid. The polysaccharide-based fracturing fluid under discussion was optimized at a lower pH range and did not break while performing compatibility tests when mixed with gelled and emulsified acids systems. The rheology and nondamaging characteristics of this polysaccharide-based fracturing fluid was measured by a high-pressure/high-temperature (HP/HT) rheometer and filter press apparatus. This fluid was optimized using a HP/HT rheometer to obtain the required break profile in accordance with the treatment design and a bottomhole temperature of up to 285°F. A stable fracturing fluid was generated by varying the crosslinker, oxidizing, and reducing agents. A comparison of high-pH guar/borate and low-pH polysaccharide-based fracturing fluids with regard to their compatibilities in both a gelled and emulsified acid system environment demonstrates the stability of the polysaccharide-based fracturing fluid in an acidic environment. To assess the nondamaging behavior of this fracturing fluid, HP/HT static filtration tests were conducted at 250°F. The fracturing fluid filter cake was prepared on a 5-μm ceramic disk and shut in for 24 hours. Filter cake and/or residue was not found on the filter disk after this experiment, indicating that the polysaccharide fluid had cleaned up completely and that minimal formation damage would be found on the formation face after an acidizing treatment. This new residue-free polysaccharide-based fracturing fluid provides improved fluid properties, as compared to a typical guar/borate crosslinked system. It does not leave insoluble residue in the formation, the fluid is robust in an acidic environment, and the fluid rheology can be precisely optimized to obtain the required break profile to optimize the fracture design as the formation dictates.